There is the adjustable backplate sold in kit form by Hemingway.

I have four chucks, two 5" Pratt Burnerd and a good Chinese 125mm and a reasonable Chinese 160mm modified to enable fine adjustment. The Pratts, unusually, have front and rear mounting holes/ threads in them, so I use six SHCS screws in them. The 125mm is rear mounting, originally three bolts, but there is plenty of space to drill and tap a second set. The 160mm has six 6mm SHCS screw in the rear because the backplate was slightly under 160mm and the original 8mm's would have been too close to the edge.

The registers in each backplate are about 0.010" undersize and the bolt holes are loose to match. Just slackening the screws a little enables the workpiece in the jaws to be set dead on with a DTI, by gently tapping the body of the chuck with a copper hammer and rechecked when the screws are retightened.

The chuck manufacturers would not recommend this as they want to sell set tru types at double the price.

Of course this only applies to scroll chucks, not four jaw independents.

Edited By old mart on 05/08/2019 16:50:07

I think your idea has merit and could be done with no alteration to the machine if you had a sub-plate between chuck and spindle register.

The thinnest arrangement would be with a front-bolted chuck to sub-plate and a chuck register larger than the spindle register (so they telescope slightly).

A rear-fixed chuck would need a thicker sub-plate and maybe bolts with holes drilled in the sides for a tommy bar (look up 'capstan nut' for the concept).

I have a cheap 3 jaw chuck that is quite accurate on some diameters of stock and much less on others, the accuracy varying between 0.001" and 0.005" T.I.R. Having checked the parallelism between register, back plate and chuck faces I come to the conclusion that the scroll is likely to be the cause of the variation so it seems that for one-offs I might as well use an independent four jaw chuck as attempt to adjust the self centering one………I am hoping to improve things now that I have discovered small burrs on the scroll Colin.

The scroll needs to be checked for radial slackness.

Edited By colin hawes on 06/08/2019 10:33:04

I second that.

The main reason for a close fitting register is to ensure the chuck body is concentric. If the chuck is a high quality one then the concentricity of what it is gripping will be as the manufacturer specified and stated on its test certificate.

Having the body concentric also ensures it will be in balance which matters especially at high spindle speeds.

With a slack register and oversize boltholes, one could set up the chuck holding a test peice and indicator, at that same diameter even a bad chuck might grip more accurately than a good one on a tight register, but there is no saying that it will be as good at other gripped diameters.

If the bolts holding the chuck to the backplate are correctly tight then I doubt and cutting force it will encounter would cause and movement.

Ian P